Gravitational wave constraints on dark sector models

TL;DR

This paper investigates how recent gravitational wave observations, especially GW170817, constrain dark sector models, using an equation of state framework to identify models that can evade these constraints while still modifying gravity cosmologically.

Contribution

It provides a model-independent analysis of gravitational wave constraints on dark sector theories, highlighting potential models that can evade current observational bounds.

Findings

Confirmed strong constraints on Horndeski, Einstein-Aether, and massive gravity models.

Identified possible models with cancellations or nonlocal features that evade GW constraints.

Discussed the role of wave number to Hubble rate ratio in suppressing gravitational wave speed modifications.

Abstract

We explore the constraints on dark sector models imposed by the recent observation of coincident gravitational waves and gamma rays from a binary neutron star merger, GW170817. Rather than focusing on specific models as has been considered by other authors, we explore this in the context of the equation of state approach of which the specific models are special cases. After confirming the strong constraints found by others for Horndeski, Einstein-Aether and massive gravity models, we discuss how it is possible to construct models which might evade the constraints from GW170817 but still leading to cosmologically interesting modifications to gravity. Possible examples are ``miracle cancellations" such as in $f(R)$ models, nonlocal models and higher-order derivatives. The latter two rely on the dimensionless ratio of the wave number of the observed gravitational waves to the Hubble expansion rate being very large ($\sim10^{19}$) which is used to suppress modifications to the speed of gravitational waves.